A significant proportion of the population is affected by respiratory disturbances. One such respiratory disturbance, is for example, sleep apnea, a temporary cessation of breathing. The most frequent occurence is obstructive sleep apnea, which affects approximately 6% of the male population over the age of 40 years. In obstructive sleep apnea, the upper air passages collapse and close, thereby preventing respiration air from passing. That can occur repeatedly in sleep.
Another form of sleep apnea is the central sleep apnea syndrome. With that syndrome, the breathing passages remain open but central control of the breathing muscles is detrimentally affected. While that form of apnea is observed in between about 10 and 20% of all persons suffering from sleep apnea, it has a high prevalence in patients suffering from cardiac insufficiency. Such patients also frequently have Cheyne-Stokes breathing, which is a periodic decrease and increase in respiration amplitude, also referred to as respiration depth. In that case, the patient has periods of reduced respiration depth, that is to say central apnea, and periods of increased respiration depth, also referred to as hyperventilation. The heart rate, hemodynamics and blood pressure are influenced by those respiration malfunctions. For example, the apnea periods can stimulate an increase in sympathetic activity, which can adversely affect the heart. The concurrence of sleep apnea with cardiac insufficiency substantially reduces the quality of life and capability of the patient. It is therefore essential that in such cases, sleep apnea is monitored, identified and treated.
In general terms, respiration malfunctions such as for example, sleep apnea, require continuous monitoring of a patient and if possible, continuous treatment, rather than only during a stay in a medical institution. In order to monitor and treat that malfunction, the patient has to be monitored and treated while asleep at home. Devices from the prior art include the use of externally applied respiration sensors and respiration masks. With those devices, sleep apnea is treated by artificial respiration devices, which control respiration and enforce inspiration and expiration. In actual fact, those devices involve a significant restriction in terms of quality of life. As that therapy depends on the co-operation of the patient, the imposing nature of that device could prevent its on-going use.
The article “Benefit of Atrial Pacing in Sleep Apnea Syndrome” by Garrigue et al, which appeared in the New England Journal of Medicine (Vol 346, No 6, pages 404-412, Feb. 7, 2002), describes a study conducted on a patient group which already had implanted cardiac pacemakers for the treatment of sinus bradycardia by means of atrial superstimulation. Based on some reports of these patients that they had fewer respiration difficulties after the implantation procedure, 15 patients were selected for investigation in a succession of nights. During the tests the implanted pacemaker was programmed to either not stimulate the heart at all or to place the heart continuously. During the stimulus phase, atrial superstimulation was set at about 15 beats per minute above the average heart rate of the patient at night. In the case of 13 out of 15 patients, the observed apnea-hypopnea index fell by more than 50% during the nights with continuous dual-chamber stimulus by the cardiac pacemaker. The apnea-hypopnea index is a measurement of the frequency of the skipped or slowed respiration rate at night. The reason for the observed improvement is not referred to, but it was possible to improve both obstructive and central apnea by means of the pacemaker equipment.
At the end of the year 2003, the St Jude Medical Center announced a new study relating to evaluation of pacemaker therapy for sleep apnea. As in the case of the Garrigue study, the plan is to assess the influence of an increased pacemaker rate during rest, but the study is evidently restricted to cardiac pacemaker patients with diagnosed sleep apnea. Evidently, the St Jude study seeks to use a secret algorithm for downloading into the cardiac pacemaker of a patient.
The use of the heartbeat volume as an input parameter is known in pacemaker therapy, for adjusting or adapting the pacemaker rate. A respiration volume value is calculated from the frequency and the relative amplitude of a reference signal, which can be determined from a measurement of an intrathoracal impedance. Evidently, the St Jude algorithm uses a time-of-day clock for switching the pacemaker therapy on and off. It will be appreciated that an algorithm based on time-of-day clock readings can cause problems if the patient travels across time zones, when there are changes in time (as for daylight savings time) and if the patient has a disruption in the normal sleep schedule.